Parents trying to determine the temperature
of a feverish child may stick a thermometer under the tongue
and under the arm and get slightly different measurements. Scientists
trying to learn if the Earth's temperature is rising also rely
on measurements taken at various places  the Earth's surface
and its atmosphere - which also often produce different readings.

In both cases, the mere fact that the measurements
in different places give slightly varied answers does not necessarily
mean that one or the other is wrong. This issue is investigated
in two papers published in the Feb. 18 issue of the journal Science.

"Scientists have puzzled over the
difference between the findings from the satellites, which measured
temperatures in the atmosphere, and those measurements that were
obtained from the surface observing systems," said Dian
Gaffen, a research meteorologist with NOAA's
Air Resources Laboratory
in Silver Spring, Md, lead author of one of the papers. Benjamin
Santer of the Lawrence
Livermore National Laboratory in Livermore, Calif., is the
lead author on the other paper; Gaffen is a contributor. Both
papers involve scientists at a number of different institutes.

"Our findings support the idea that
the difference is largely real and that it may be due to a combination
of natural and manmade factors," said Santer.

Gaffen and colleagues used data from radiosondes
(weather balloons carrying instrument packages) to obtain independent
data on both surface and lower-troposphere temperature change.
The troposphere is the area of the atmosphere where weather occurs,
extending about seven miles from the Earth's surface to the next
layer, the stratosphere. Their findings show that the temperature
trends are consistent with the satellite and surface results
since 1979.

"Previous work with the satellite
data suggested little or no temperature trend in the past twenty
years, while the surface data show marked warming. We find that
a third observing system, radiosondes, shows the same pattern
as the satellite and surface data in the tropics, where the surface
and tropospheric temperature change show the largest differences,"
said Gaffen. "The radiosondes also show that the tropical
atmosphere has become slightly more unstable since 1979, when
the satellite data began. But when we look further back, to 1960,
we see more consistent warming at the surface and in the lower
troposphere, which tells us that the past two decades might not
be representative of longer-term changes."

In the companion paper, Santer and colleagues
found that the discrepancies can partially be attributed to the
fact that the surface observing system does not cover some parts
of the globe, unlike the satellite system. Computer model simulations
of the climate system indicate that the remaining differential
cannot be explained by the natural variability of the climate
system or by climate change resulting from increases in greenhouse
gases alone.

The Santer et al. paper shows that different
climate "forcing factors" may have had quite different
influences on surface and atmospheric temperature. Increases
in greenhouse gases probably act to warm the troposphere more
than the surface. In contrast, the combined effects of aerosols
from major volcanic eruptions (such as that of Mt. Pinatubo in
1991) and human-caused decreases in stratospheric ozone may act
to cool the troposphere more than the surface.

Climate models have often been criticized
for their failure to reproduce warming of the surface relative
to the troposphere. Such criticism is usually based on model
experiments involving changes in greenhouse gases alone. "Our
work shows that the correspondence between modeled and observed
temperature changes is much closer if the model experiments include
a combination of human-caused and natural climate effects over
the past twenty years, and not just changes in greenhouse gases,"
said Santer.

Similar issues were addressed in a recent
report of the National
Research Council. Gaffen and Santer were part of the 11-member
panel that dealt with the issue of why the satellite data apparently
show little or no warming of the troposphere, while ground-based
thermometers indicated marked warming of the Earth's surface.
The panel concluded that this discrepancy "in no way invalidates
the conclusion that the Earth's temperature has been rising."
The research described in both Science papers supports
this conclusion.